Environmental Problems, Their Causes, and Sustainability 1

suited for plant growth. It has a texture between theseextremes—a crumbly, spongy feeling—with many ofits particles clumped loosely together.Soil texture helps determine soil porosity, a measureof the volume of pores or spaces per volume ofsoil and of the average distances between those spaces.Fine particles are needed for water retention andcoarse ones for air spaces. A porous soil has manypores and can hold more water and air than a lessporous soil. The average size of the spaces or pores in asoil determines soil permeability: the rate at whichwater and air move from upper to lower soil layers.4-7 MATTER CYCLING IN ECOSYSTEMSWhat Are Biogeochemical Cycles? Going inCirclesGlobal cycles recycle nutrients through the earth’s air,land, water, and living organisms and, in the process,connect past, present, and future forms of life.All organisms are interconnected by vast global recyclingsystems made up of nutrient cycles, or biogeochemicalcycles (literally, life–earth–chemical cycles).In these cycles, nutrient atoms, ions, and moleculesthat organisms need to live, grow, and reproduce arecontinuously cycled between air, water, soil, rock, andliving organisms. These cycles, driven directly or indirectlyby incoming solar energy and gravity, includethe carbon, oxygen, nitrogen, phosphorus, and hydrologic(water) cycles (Figure 4-8).The earth’s chemical cycles connect past, present,and future forms of life. Some of the carbon atoms inyour skin may once have been part of a leaf, a dinosaur’sskin, or a layer of limestone rock. Your grandmother,Plato, or a hunter–gatherer who lived 25,000years ago may have inhaled some of the oxygen moleculesyou just inhaled.How Is Water Cycled in the Biosphere?The Water CycleA vast global cycle collects, purifies, distributes, andrecycles the earth’s fixed supply of water.The hydrologic cycle, or water cycle, recycles theearth’s fixed supply of water, as shown in Figure 4-28.Trace the flows and paths in this diagram. Solar energyevaporates water found on the earth’s surface into theatmosphere. Some of this water returns to the earth asrain or snow, passes through living organisms, flowsinto bodies of water, and eventually is evaporatedagain to continue the cycle. The water cycle differsfrom most other nutrient cycles in that most of the waterremains chemically unchanged and is transformedfrom one physical state to another.Rain cloudsCondensationPrecipitationPrecipitationto landRunoffTranspirationTranspirationfrom plantsSurface runoff(rapid)Evaporationfrom landEvaporationEvaporationfrom oceanPrecipitationPrecipitationto oceanInfiltration andPercolationSurfacerunoff(rapid)Groundwater movement (slow)Ocean storageFigure 4-28 Natural capital: simplified model of the global hydrologic cycle that helps keep you alive.76 CHAPTER 4 Ecosystems: What Are They and How Do They Work?